The present invention relates to the manufacture of thin film solid state electrochemical devices using solvent-free polymerization of polymer solid electrolytes and redox polymer cathode materials.
Solvent-free polymer electrolytes have generated significant interest in recent years, primarily due to the potential for producing thin film rechargeable lithium batteries with high power capability and energy density. They have been extensively described in the patent literature, e.g. U.S. Pat. Nos. 4,303,748 to Armand, et al.; 4,589,197 to North; 4,547,440 to Hooper, et al.; 4,906,718 to Gornowicz, et al. and 4,228,226 to Christiansen. The cells are construed from an alkali metal foil anode, an ionically conducting polymer solid electrolyte containing an alkali metal foil anode, an ionically conducting polymer solid electrolyte containing an alkali metal salt, and a cathode consisting of a composite of a powdered insertion material, such as TiS.sub.2, the polymer electrolyte and an electron conductor, such as finely dispersed carbon black. Like liquid electrolytes and solvent-swollen polyelectrolytes used in ion-exchange resins, solvent-free polymer electrolytes possess ion transport properties. Both cation transport and anion transport in these solid polymer electrolytes have been substantiated and are well documented in the prior art.
In applications of solid polymer electrolytes to secondary solid state batteries, it would be preferable to have no anion migration, the result of which is less polarization and higher power output. Anion mobility produces a negative effect on the energy efficiency of the battery because it results in local concentration gradients which result in deleterious polarization of the cell, lowering the output current.
Attempts have been made to immobilize the anion on the polymer chain in order to achieve specific cationic conductivity. Several approaches have included the synthesis of cationic single-ionic conductors based on carboxylate or sulfonate salts. (Tsuchida et al, Macromolecules 21,96(1988)). These reported electrolytes are limited in their application due to low conductivity. Presumably, the low conductivity is due to the extensive ion pairing in these salts. High conductivity polymer solid electrolytes with specific cation conductivity have been synthesized by Skotheim et al (U.S. Pat. No. 4,882,243 (1989)) where the immobilized anionic moieties are based on sterically hindered phenol compounds. Sterically hindered phenol substituted polysiloxanes have demonstrated specific cation conductivity 100-1000 times higher than what has been previously achieved with covalently attached carboxylate or sulfonate salts.
Polymer solid electrolytes are generally cast from a common organic solvent for the polymer and the alkali metal salts, such as methanol or acetonitrile. Disposing of the organic solvent poses an environmental hazard and adds a considerable manufacturing cost. It would be preferable to synthesize the polymer electrolyte using a solvent-free polymerization method where the polymerization is performed under actinic irradiation. Actinic irradiation is defined as ultraviolet, gamma ray or electron beam irradiation.
M.-T. Lee et al., U.S. Pat. No. 4.830,939, describes a method for forming an interpenetrating polymeric network for use in solid state electrochemical cells, consisting of a liquid electrolyte trapped in a crosslinked polymer matrix. The two phase polymer electrolyte system is formed by subjecting a mixture consisting of a liquid monomeric or prepolymeric radiation polymerizable compound, a radiation inert ionically conducting compound, such as propylene carbonate (PC), and an alkali metal salt, such as lithium trifluoromethane sulfonate, to actinic radiation to thereby crosslink the radiation polymerizable material and form a solid matrix.
Electrochemically, the composite electrolyte material described by Lee et al. behaves essentially as a liquid electrolyte, with the well known degradation problems associated with liquid electrolytes. The long term stability of liquid electrolyte based electrochemical cells is limited by corrosion at the electrode/electrolyte interface, leading tto the build up of passivating layers on the lithium electrode. In addition, with liquid electrolytes, co-insertion of the solvent and the alkali metal cation, e.g. lithium, into the cathode material results in degradation of the cathode material due to swelling and de-swelling upon discharging and charging of the battery. It would be preferable to use high conductivity polymer electrolytes containing no liquid components.
The cathode materials used in manufacturing of thin flim lithium or sodium batteries have generally been intercalation compounds such as TiS.sub.2 and V.sub.6 O.sub.13. The cells have had limited rate capability and low utilization of cathode capacity. The cathode is formed as a composite consisting of powdered intercalation material and the polymer electrolyte with finely dispersed carbon black as electrical conductor. The rate limiting factor is generally the diffusion of cation in the insertion host material. Recently, M. Liu et al (Proc. Electrochem. Soc. Meeting, Miami, Fla., Sep. 1989) have describe a new class of redox polymer based cathode materials with substantially improved rate capability for lithium or sodium secondary batteries. The materials are based on polymerization and depolymerization via sulfur-sulfur bonds during the charging and discharging of the battery. Electrochemical cells were made with lithium foil anode, an electrolyte consisting of polyethylene oxide (PEO) with a lithium salt, such as LiClO.sub.4 or LiSO.sub.3 CF.sub.3, and a cathode consising of a homogeneous mixture of PEO and a redox polymer, with added carbon black for electrical conductivity. The cells demonstrated considerably higher rate capabilities than comparable cells made with TiS.sub.2 cathodes.
One drawback with the system described by Liu et al is the reliance on polymerization and depolymerization of the redox cathode. When depolymerized, the monomers could disperse into the polymer electrolyte over time, severly limiting the lifetime of the cell. A second problem arises from the polymer electrolyte and the redox polymer cathode having different polymeric backbones. The miscibility of different polymers is a well known problem. With different polymeric systems, phase segregation normally occurs. Basing the electrolyte and the electrode materials on the same polymeric backbone and employing high conducting single-ion conducting polymer electrolytes would be expected to result in improved long term stability and higher capacity.